More than 10 types of antibody are currently marketed in the U.S.A. as medicaments. Antibody drugs are attracting attention as molecular targeted drugs that can provide the highest certainty, contributing to the rapid enlargement of a field of new medicaments. Most antibody drugs that are currently under development or in use utilize immunoglobulin G (IgG) class antibodies. To date, an anti-Fc antibody or Protein A/G that specifically binds to the Fc region of an IgG antibody has often been used for detection or purification of an IgG antibody (Non-patent Documents 1 and 2). However, an anti-Fc antibody prepared by hybridoma technology or animal immunization has drawbacks such that it is not only expensive, but is also easily denatured upon labeling, for example. On the other hand, Protein A/G has drawbacks such that it is does not allow one to distinguish between IgGs derived from different organism species and it does not bind to human IgG3. Protein A/G is generally isolated from bacteria, leading to possible contamination with an endotoxin such as LPS. This poses a big problem upon the use of a Protein A column for antibody drug purification. Furthermore, antibody purification using a Protein A column can also lead to a problem such that antibody denaturation takes place due to acid when the antibody is eluded from the column using an acid solution, so that the yield drastically decreases or the quality of the thus purified antibody is lowered. It is very important to overcome such problems with antibody purification methods using an anti-IgG antibody or Protein A/G so as to develop a novel technique capable of specifically detecting and purifying human IgG.
Many IgG binding peptides that can be used for IgG purification have been reported so far. Fassina et al., have screened synthetic multimeric peptide libraries and then identified a Protein A peptide mimic that competes with Protein A for interactions with biotin-labeled immunoglobulin (Non-patent Documents 3 and 4). Also, Ehrlish et al., have isolated an Fc-binding peptide having a Protein A mimic sequence from an M13 phage display library displaying 12-mer or 7-mer linear peptides (Non-patent Document 5). Krook et al., have also reported identification of a Protein A peptide analog via reaction of a random peptide fUSE5 phage library displaying linear 10-mer peptides with IgG followed by elution with the use of Protein A (Non-patent Document 6). In 2005, Verdoliva et al., have reported a peptide motif from a cyclic synthetic peptide library, that recognizes regions near the hinge region of an IgG antibody and inhibits competitively the reaction of FcγRIII with IgG Fc (Non-patent Document 7). Moreover, Suzuki et al., have described peptides having a property of binding to IgG Fc fragments and disclosed a method for detecting IgG using the same (Patent Document 1). However, it cannot be said that all of these peptide motifs have sufficient IgG binding affinity as tags for purification and/or detection of an IgG antibody.
Meanwhile, in 2000, DeLano et al. have reported polypeptide sequences having binding affinity for the hinge region on a human IgG Fc fragment, such as the Fc-III peptide (DCAWHLGELVWCT; SEQ ID NO: 15), as a result of the use of an M13 phage library displaying cyclic peptides (Non-patent Document 8 and Patent Document 2). Furthermore, in 2006, Dias et al., have performed engineering so as to further stabilize the circularization of the peptide Fc-III (Non-patent Document 9). The peptide Fc-III of Delano et al., exhibited its binding affinity with a Kd value of 185 nM for an IgG antibody, but the Kd value exhibited by the improved peptide FcBP-2 of Dias et al. decreased to approximately 2 nM. It was thus suggested that the peptide FcBP-2 had an extremely strong binding force. However, the dissociation rate constant, koff, of FcBP-2 was found to be 10−2 sec−1, as a result of analysis of kinetic parameters. Such dissociation rate is significantly faster than that of general koff (10−3 to 10−5 Sec−1) of an antibody. This indicates that even if the peptide FcBP-2 is used as a tag for purification or detection of an antibody, the dissociation rate is so fast that the bond between the peptide FcBP-2 and the antibody dissociate rapidly and the bond cannot be retained. Accordingly, FcBP-2 is inappropriate for use in human IgG purification and/or detection.
Patent Document 3 discloses various human IgG Fc binding peptides. However, Patent Document 3 does not disclose peptides appropriate for human IgG purification and/or detection, such as peptides capable of sufficiently retaining the bonds with any human IgG subclass and exhibiting no significant binding to IgGs of other organism species.    Patent Document 1: JP Patent Publication (Kokai) No. 2004-187563    Patent Document 2: International Publication WO01/045746 Pamphlet    Patent Document 3: International Publication WO02/086070 Pamphlet    Non-patent Document 1: Ey P. L., et al., Immunochemistry (1978) 15, p. 429-436    Non-patent Document 2: Akerstrom B., et al., J. Immunol., (1985) 135, p. 2589-2592    Non-patent Document 3: Fassina G., et al., J. Mol. Recognit. (1996) 9, p. 564-569    Non-patent Document 4: Fassina G., et al., J. Mol. Recognit. (1998) 11, p. 128-133    Non-patent Document 5: Ehrlich G. K. and Bailon P., J. Mol. Recognit. (1998) 11, p. 121-125    Non-patent Document 6: Krook M., Mosbach K., and Ramstrom O., J. Immunol. Methods (1998) 221, p. 151-157    Non-patent Document 7: Verdoliva A. et al., Chembiochem (2005) 6, p. 1242-1253    Non-patent Document 8: DeLano W. L., et al., Science (2000) 287, p. 1279-1283    Non-patent Document 9: Dias R. L., et al., J. Am. Chem. Soc. (2006) 128, p. 2726-2732